This invention relates to load control or demand response by a utility; and more particularly to apparatus and a method for detecting an under-frequency or under-voltage condition on the utility's power distribution system which indicates need for load control. An over value detection capability is also provided in accordance with the method.
An electrical utility supplies power to a variety of customers over a wide geographic area. The power commonly supplied by the utility is three-phase (3φ), 60 cycle (Hz), 120 volt (VAC.) power. Under normal operating conditions, the power supplied is uniform throughout the system. At these times, the system effectively distributes power to a wide range of electrical loads all of which can operate at 100% of their capabilities.
Sometimes, however, circumstances occur in which excessive load on the system impacts the quality of electrical service provided by the utility. An exemplary cause of this is high heat and high humidity which occurs during summer months and during which times electrical equipment such as air conditioners and dehumidifiers are constantly running. The effect created during these peak times of electrical usage manifest themselves by the line frequency and/or line voltage falling below the normal levels noted above. When this happens, unless the utility can control consumption (i.e., shed load), blackouts or brownouts may occur.
It is known in the art to connect power management devices, commonly referred to as load control or demand response units (LCUs or DRUs) to appliances or other electrical loads at a customer's location. See, for example, U.S. Pat. No. 7,355,301 (the '301 patent), U.S. Pat. No. 7,242,114 (the '114 patent), U.S. Pat. No. 7,149,605 (the '605 patent) and U.S. Pat. No. 7,010,363, and published U.S. patent application 2006/0229768. The device may be connected to a single load, or it may be connected to a number of loads.
As described in the '301 patent, a primary voltage source is sampled at regular intervals. The resulting voltage readings are then compared to voltage readings representing an under-frequency or under-voltage threshold. When an under-frequency or under-voltage condition is detected, an in-response cycle is commenced during which the electrical load imposed on the system is controlled. When the readings fall below a “fail” level, load restore counter values are stored in a memory before load is shed. A restore load response is initiated when the voltage reading values rise above a “restore” level and remain above that level for a period of time.
As taught in the '605 patent, for example, a power management device includes control circuitry that monitors received electrical energy (AC line waveform) and controls operations of loads so to maintain operation of a utility's power distribution system within a desired range. This includes adjusting the amount of electrical energy supplied to respective loads. In this regard, control circuitry transmits a control signal to a controller for a respective load, with the control signal causing the load on the system to be reduced. Alternatively, the control circuitry operates a control relay to adjust the amount of electrical energy applied to the load. The control circuitry can completely shut down a load.
Incumbent upon the operation of any load control device is the ability to first detect the occurrence of an under-voltage or under-frequency condition so demand control protocols can be implemented; and second, to determine when the voltage or frequency has returned to within their normal range so load control can be terminated. In the power management device described in the '605 patent, a step-down transformer reduces line voltage to an AC voltage of significantly less peak amplitude and the AC voltage waveform is converted to a square wave. A voltage clipping circuit processes the waveform to produce a pulse waveform whose leading (rising) edge represents a positive zero-crossing and whose trailing (falling) edge represents a negative zero-crossing. For monitoring line frequency, a reference clock signal is supplied to a control circuit which counts the number of clock signal pulses between (successive) rising edges of the pulse waveform. The resulting count represents the line frequency of electrical energy transmitted by the utility. The control circuit accesses a count value representing a load shed threshold and compares the number of counted clock pulses to the threshold value. If the threshold value is exceeded, load shedding is commenced and continues until the count value representing line frequency is again less than the threshold value.
A different process is described in the '114 patent in which clock pulses are again counted to determine line frequency with a counter being incremented or decremented in response to a count value. As described in the '114 patent, when line frequency is 60 Hz, a count value is initialized to zero. If the line frequency then falls below a preset frequency value, the counter starts counting up. If the line frequency then climbs back above the threshold, the counter starts counting back down to zero. However, if the count value exceeds an under-frequency count value, a line under frequency (LUF) condition is considered to have been detected. This condition remains in effect until a restore condition is satisfied based upon the count value again going to zero.
In both the '605 and '114 patents, the method of determining an under-voltage condition is similar to that outlined above with respect to determining an under-frequency condition. While the methods described in these patents may be effective for their intended purposes, another, simpler to implement method has been developed which accurately determines when an under-frequency or under-voltage condition exists, for providing load control throughout the duration of the situation, and for accurately determining when the condition is over and load control is no longer necessary.